1. Field of the Invention
The invention relates to an exhaust gas control apparatus for an internal combustion engine, and more specifically to an exhaust gas control apparatus for an internal combustion engine provided with a NOx storage/reduction catalyst.
2. Description of the Related Art
A NOx storage/reduction catalyst is known which stores NOx in the exhaust gas by at least one of adsorption and absorption when the air-fuel ratio of in-flowing exhaust gas is lean, and then reduces and purifies the stored NOx using, for example, HC and reduction components such as CO and H2 (hereinafter these will be collectively termed “reduction components”) in the exhaust gas when the air-fuel ratio of the in-flowing exhaust gas is rich.
One such exhaust gas control apparatus for an internal combustion engine that uses this type of NOx storage/reduction catalyst is disclosed in Japanese patent laid open application No. JP(A) 2000-154713.
The apparatus disclosed in this publication improves the NOx purification efficiency of the NOx storage/reduction catalyst by carrying an oxygen storage component on only the front half of a carrier of the NOx storage/reduction catalyst.
When a three-way catalyst is provided in an exhaust passage on the upstream side of the NOx storage/reduction catalyst and that three-way catalyst has an oxygen (O2) storage function, the exhaust gas control performance of the NOx storage/reduction catalyst may decline due to a delay in the change in the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst.
As is well known, a three-way catalyst can be made to have an oxygen storage function by carrying on it metal components such as cerium Ce as an auxiliary agent, in addition to a precious metal catalyst component such as platinum Pt, palladium Pd, or rhodium Rh. That is, cerium carried on a catalyst as an additive agent stores oxygen by bonding to the oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is higher than the stoichiometric air-fuel ratio (i.e., when the air-fuel ratio of the exhaust gas is lean) to produce ceria (cerium oxide IV: CeO2). Further, when the air-fuel ratio of the in-flowing exhaust gas is equal to, or less than, the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is rich), the ceria releases the oxygen to produce cerium oxide III (Ce2O3).
Therefore, in three-way catalyst having an O2 storage function, oxygen is released from the three-way catalyst when the air-fuel ratio of the exhaust gas changes from lean to rich. Even if the air-fuel ratio of the exhaust gas flowing into the three-way catalyst changes to rich, the air-fuel ratio of the exhaust gas passing through the three-way catalyst is maintained near the stoichiometric air-fuel ratio while oxygen is being released from the three-way catalyst.
When the three-way catalyst provided in an exhaust passage on the upstream side of the NOx storage/reduction catalyst has an O2 storage function, even if the air-fuel ratio of the exhaust gas from the engine changes from lean to rich during a rich spike operation of the engine, the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst does not immediately become rich, but rather is temporarily maintained near the stoichiometric air-fuel ratio. That is, the reduction components in exhaust gas having a rich air-fuel ratio are oxidized by the oxygen released from the oxygen storage component of the catalyst, such that exhaust gas with an air-fuel ratio near the stoichiometric air-fuel ratio and which contains relatively few reduction components flows into the NOx storage/reduction catalyst.
Meanwhile, NOx is released from the NOx storage/reduction catalyst when the air-fuel ratio of the exhaust gas changes (drops) from a lean air-fuel ratio to an air-fuel ratio near the stoichiometric air-fuel ratio, but the exhaust gas that flows into the NOx storage/reduction catalyst contains only a relatively small amount of reduction components, not enough to reduce the entire amount of NOx that is released. As a result, the NOx that was released from the NOx storage/reduction catalyst and not reduced may flow out from the downstream side of the NOx storage/reduction catalyst.
Because of this, the apparatus disclosed in Japanese laid open application No. JP(A) 2000-154713 improves the NOx purification efficiency of the NOx storage/reduction catalyst by applying the O2 storage function to the front half portion of the NOx storage/reduction catalyst or providing a three-way catalyst having an O2 storage function adjacent to, and on the upstream side of, the NOx storage/reduction catalyst.
Accordingly, by providing the three-way catalyst having an O2 storage function on the upstream side of the NOx storage/reduction catalyst in this apparatus, the reduction components in the exhaust gas are oxidized by oxygen released from the ceria when the air-fuel ratio of the in-flowing exhaust gas is rich. Reaction heat from that reaction raises the temperature of the NOx storage/reduction catalyst component carried on the carrier, which promotes the release of NOx from the NOx storage/reduction catalyst and improves the catalyst activity. This is believed to increase the purification efficiency of the released NOx.
As described above, the apparatus disclosed in Japanese patent laid open application No. JP(A) 2000-154713 improves the purification efficiency of the catalyst by carrying the oxygen storage component on only the front half of the carrier of the NOx storage/reduction catalyst.
However, while having the oxygen storage component carried on only the front half of the catalyst carrier is beneficial for improving catalyst activity by raising the temperature of the catalyst by oxidizing the HC and CO in the exhaust gas when the air-fuel ratio is rich, as described in Japanese patent laid open application No. JP(A) 2000-154713, problems still remain. That is, during the initial period of a change from a lean air-fuel ratio to a rich air-fuel ratio, the reduction components in the exhaust gas end up becoming oxidized by the oxygen released from the oxygen storage component, which results in a shortage of reduction components for reducing and purifying the NOx stored in the NOx storage/reduction catalyst.
On the other hand, in order to solve this problem, it is also possible not to have the oxygen storage component be carried on (either the front half or the back half) of the NOx storage/reduction catalyst carrier, as is done in the related art. However, the NOx storage/reduction catalyst functions as a three-way catalyst that simultaneously purifies three components (HC, CO, NOx) in the exhaust gas in a narrow air-fuel ratio range near the stoichiometric air-fuel ratio. Therefore, when the engine is operated near the stoichiometric air-fuel ratio, in order to eliminate relatively small air-fuel ratio fluctuations near the stoichiometric air-fuel ratio of the exhaust gas and effectively utilize the three-way catalyst function of the NOx storage/reduction catalyst, it is necessary to carry at least a certain amount of the oxygen storage component on the carrier of the NOx storage/reduction catalyst.
That is, even though carrying the oxygen storage component on the NOx storage/reduction catalyst carrier is problematic from the viewpoint of NOx control, in reality it is necessary to carry the oxygen storage component in order to make use of the three-way catalyst function.